This study presents the results of the 2013 Ibiza (Western Mediterranean) calibration campaign of Jason-2 and SARAL altimeters. It took place from 14 to 16 September 2013 and was composed of two phases: the calibration of the GNSS (Global Navigation Satellite System) buoys to estimate the antenna height of each of them and absolute calibration to estimate the altimeter bias (i.e., the difference of sea level measured by radar altimetry and GNSS). The first one was achieved in the Ibiza harbor at a close vicinity of the Ibiza tide gauge and the second one was at ˜ 40 km at the northwest of Ibiza Island at a crossover point of Jason-2 and SARAL nominal groundtracks. Five buoys were used to delineate the crossover region and their measurements interpolated at the exact location of each overflight. The overflights occurred two consecutive days: 15 and 16 September 2013 for Jason-2 and SARAL respectively. The GNSS data were processed using precise point positioning technique. The biases found are of (-0.1 ± 0.9) and (-3.1 ± 1.5) cm for Jason-2 and SARAL respectively.

In this paper we describe the statistical properties of the EUV solar flux sudden variation. The solar flux variation is modeled as a time series characterized by the subsolar Vertical Total Electron Content double difference in time, computed with dual-frequency GNSS (Global Navigation Satellite Systems) measurements in the daylight hemisphere (GNSS solar flare indicator rate parameter). We propose a model that explains its characteristics and the forecasting limitations. The sudden overionization pattern is assumed to be of solar origin, and the data used in this study was collected during the last solar cycle. The two defining characteristics of this time series are an extreme variability (i.e., in a solar cycle one can find events at 400 sigma from the mean value) and a temporal correlation that is independent of the timescale. We give a characterization of a model that explains the empirical results and properties such as (a) the persistence and presence of bursts of solar flares and (b) their long tail peak values of the solar flux variation. We show that the solar flux variation time series can be characterized by a fractional Brownian model for the long-term dependence, and a power law distribution for the extreme values that appear in the time series.

An altimetry calibration campaign was achieved in the Mediterranean Sea, close to the Ibiza island (Baleares)
area, last September in the framework of a Spanish-French cooperation. Its goal was to provide absolute biases for
the Jason-2 and AltiKa/Saral altimeters through comparisons with GNSS measurements on buoys. A similar
Spanish/French experiment was already performed for Jason-1in June 2003 in this geographical area under the
name IBIZA 2003 campaign.
Direct absolute altimeter calibration, estimating the Jason-2 and AltiKa/Saral biases, was made from direct
overflights using GPS buoys. This method does not require any modelling of geoid and tidal error. The crossover
point between Jason-2 and Saral North of Ibiza (around 40 nm) and West of Mallorca island was found to be
optimal for our purposes as it allows measurements at a one-day time-lag and a similar configuration of buoys for
each satellite pass. 5 buoys were deployed near a Jason-2/AltiKA Saral crossover point to determine the sea
surface in the along-track and cross-track directions, to estimate by interpolation the exact nadir point of the
satellite. Here, we present the experimental settings of the campaign and the datasets used in this study, the
methods used for comparing altimetry data with GNSS measurements, and the first results of the absolute
calibration.

In this paper we describe the statistical properties of the EUV solar flux sudden variation. The solar flux variation is modeled as a time series characterized by the subsolar VTEC (Vertical Total Electron Content) double-difference in time, computed with dual frequency GNSS (Global Navigation Satellite System) measurements in the daylight hemisphere. By assuming a sudden overionization pattern of solar origin, during the last solar cycle, we propose a model that explains it's characteristics, and the forecasting limitations. The two defining characteristics of this time series, is an extreme variability (i.e.\ in a solar cycle one can find events at $400 \sigma$ from the mean value) and a temporal correlation that is independent of the time scale. We give a characterization of a model that explains the empirical results, and properties such as, a) the persistence and presence of bursts of solar flares, b) their long tail peak values of the solar flux variation. We show that the solar flux variation time series can be characterized by a fractional Brownian model for the long term dependence, and a powerlaw distribution for the extreme values that appear in the time series.

A retrospective study of the asymmetry in the ionosphere during the solstices is made using the different geospace parameters in the North and South magnetic hemispheres. Data of total electron content (TEC) and global electron content (GEC) produced from global ionospheric maps, GIM-TEC for 1999-2013, the ionospheric electron content (IEC) measured by TOPEX-Jason 1 and 2 satellites for 2001-2012, the F-2 layer critical frequency and peak height measured on board ISIS 1, ISIS 2, and IK19 satellites during 1969-1982, and the earthquakes M5+ occurrences for 1999-2013 are analyzed. Annual asymmetry is observed with GEC and IEC for the years of observation with asymmetry index, AI, showing January > July excess from 0.02 to 0.25. The coincident pattern of January-to-July asymmetry ratio of TEC and IEC colocated along the magnetic longitude sector of 270 degrees +/- 5 degrees E in the Pacific Ocean is obtained varying with local time and magnetic latitude. The sea/land differences in the F-2 layer peak electron density, NmF2, and the peak height, h(m)F(2), gathered with topside sounding data exhibit tilted ionosphere along the seashores with denser electron population at greater peak heights over the sea. The topside peak electron density NmF2, TEC, IEC, and the hemisphere part of GEC are dominant in the South hemisphere which resembles the pattern for seismic activity with dominant earthquake occurrence in the South magnetic hemisphere. Though the study is made for the hemispheric and annual asymmetry during solstices in the ionosphere, the conclusions seem valid for other aspects of seismic-ionospheric associations with tectonic plate boundaries representing zones of enhanced risk for space weather.

Higher-order ionospheric effects (I2+) are one of the main limiting factors in very precise Global Navigation Satellite Systems (GNSS) processing, for applications where millimeter accuracy is demanded. This paper summarizes a comprehensive study of the I2+ effects in range and in GNSS precise products such as receiver position and clock, tropospheric delay, geocenter offset, and GNSS satellite position and clock. All the relevant higher-order contributions are considered: second and third orders, geometric bending, and slant total electron content (dSTEC) bending (i.e., the difference between the STEC for straight and bent paths). Using a realistic simulation with representative solar maximum conditions on GPS signals, both the effects and mitigation errors are analyzed. The usage of the combination of multifrequency L band observations has to be rejected due to its increased noise level. The results of the study show that the main two effects in range are the second-order ionospheric and dSTEC terms, with peak values up to 2 cm. Their combined impacts on the precise GNSS satellite products affects the satellite Z coordinates (up to +1 cm) and satellite clocks (more than ±20 ps). Other precise products are affected at the millimeter level. After correction the impact on all the precise GNSS products is reduced below 5 mm. We finally show that the I2+ impact on a Precise Point Positioning (PPP) user is lower than the current uncertainties of the PPP solutions, after applying consistently the precise products (satellite orbits and clocks) obtained under I2+ correction

The 2013 Balearic campaign GNSS position analysis of the 2013 will be performed with different softwares by different groups (similarly as it is being done in
the International GNSS Service for their different products), in order improve the high demanded accuracy for JASON2 and SARAL altimeters precise calibration.
In particular JPL GIPSY-OASIS software will be used, with the undifferenced PPP ambiguity fixing strategy. In order to improve the results accuracy, two similar
networks are being processed. The first network includes the deployed GNSS receivers and the reference stations. The second one is a control network, defined
by using the permanent receivers in the California dense network with a similar distribution as the main altimeter campaign network. In this case, the position of
the receivers plying the role of buoys are being processed in the same kinematic way than the actual buoys, in order to compare them with the very accurate
positions obtained with GIPSY-OASIS static processing.

In this work, an ionospheric activity indicator is defined based in the “weighted” Along Arc TEC rate (AATR). It is shown that this indicator, which can be easily computed from the GPS carrier phases, is well correlated with the ionospheric activity and, unlike other global indicators linked to the geomagnetic activity, can be sensitive to regional behaviours of ionospheric activity

The significance of higher order ionospheric
terms (I2+) and the performance of
realistic mitigation strategies are analyzed. After confirming the unfeasibility of removing second order ionospheric term (I2) with combinations of actual data of three -
frequency measurements (in coincidence with the theoretical expectations), we focus on
the I2+ correction modelling from electron density and geomagnetic models, and empirical bending approximations.
Using realistic simulated GNSS global observations with actual geometry (by using the IRI2012 and IGRFv11 for electron density and geomagnetic field), the impact of I2+ on high precision GNSS processing has been
quantified by using independently the GIPSY and BERNESE software which provides equivalent results. The main general conclusion is that the modeling of I2+ (mainly in terms of I2, the predominant term at all the elevations, and bending corrections) can mitigate most of I2+ signature in GNSS precise geodetic products (receiver position, clock and zenith non - hydrostatic delays, satellite orbits and clocks, and geocenter estimate), which, otherwise can be significant from mm to cm level during solar cycle maximum conditions.
A similar conclusion can be extended consistently to Precise Point Positioning (PPP)
processing.

This two-volume book contains a self-learning course and software tools aimed at providing the necessary background to start work in an operative way in GNSS navigation. The books are focused on the instrumental use of concepts and techniques involved in GNSS navigation and include all the elements needed to understand how the system works and how to work with it. After working through the two volumes, students should be able to develop their own tools for high-accuracy navigation, implementing the algorithms and expanding the skills learned.
The first volume is devoted to theory, providing a summary of GNSS fundamentals and algorithms. The second volume is devoted to laboratory exercises, with a wide range of selected practical examples going further into the theoretical concepts and their practical implementation. The exercises have been developed with a specialised software package (the ESA/U PC gLAB educational SW, on an attached CD) and selected data files are provided for the laboratory sessions.

This two-volume book contains a self-learning course and software tools aimed at providing the necessary background to start work in an operative way in GNSS navigation. The books are focused on the instrumental use of concepts and techniques involved in GNSS navigation and include all the elements needed to understand how the system works and how to work with it. After working through the two volumes, students should be able to develop their own tools for high-accuracy navigation, implementing the algorithms and expanding the skills learned.
The first volume is devoted to theory, providing a summary of GNSS fundamentals and algorithms. The second volume is devoted to laboratory exercises, with a wide range of selected practical examples going further into the theoretical concepts and their practical implementation. The exercises have been developed with a specialised software package (the ESA/U PC gLAB educational SW, on an attached CD) and selected data files are provided for the laboratory sessions.
This is an end-to-end GNSS course addressed to all professionals and students who wish to undertake a deeper study of satellite navigation, targeting the GNSS data processing and analysis issues.

The availability of representative data samples, models and statistical information derived from the analysis of parameters that adequately describe the state of the ionosphere are key elements for the design, verification and qualification of EGNOS algorithms. As part of the on-going evolutions of EGNOS V2 releases, and also in
support to the development of a future multi-constellation and dual-frequency SBAS (EGNOS V3), ESA has started a refinement process of the tools, data and precise definition of the ionospheric operational conditions. A particular driver for this process is the improvement of the availability performance of the current and future system especially under solar max and severe ionospheric conditions, without
degradation of integrity and accuracy performances. The paper provides an overview about the concept and the data and models used for EGNOS development and qualification. It outlines the approaches to generate reference models and describes the parameter used for the characterization of nominal and perturbed ionospheric conditions from an SBAS perspective.

A simple and precise technique to measure the sudden Extreme Ultraviolet (EUV) radiation increase of the Sun, during mid and strong flares, has been formulated and demonstrated for the most active part of the last Solar Cycle. On the one hand, it is based on the short time scale of these events, which allows the validity of a simple global overionization model. And on the other hand on the prompt ionospheric response to the EUV ionization, which signature (in terms of the Global Navigation Satellite Systems -GNSSSolar Flare Activity Indicator, GSFLAI) is accurately measured in real-time from the existing global networks of dual-frequency GNSS receivers, and with a time resolution higher that those of dedicated space probes. Moreover the sensitivity of this approach enables the detection of not only extreme X-class flares, but also the detection of variations of one order of magnitude lower or even smaller (such as for M-class flares): 100% successful detection for all the X-class solar flares during 2000-2006 with registered location outside of the solar limb (i.e. detection of 94% of all of X-class solar-flares) and about 65% for M-class ones, obtained with the associated SISTED detector. In summary, its full availability, continuity, high precision and integrity for mid and high solar flare effects on Ionosphere, can make GSFLAI an useful indicator of potential Space Weather activity for many users in radio propagation. These results, which have been recently published by the authors, are extended in this work up to one complete solar cycle, and using a lower sampling rate (30 seconds), demonstrating the better behaviour of this indirect solar EUV variability proxy (GSFLAI30), when comparisons are made with direct measurements from space probes, providing readings which can be affected by the late enhancement of particles.

A simple model for the topside ionosphere region is introduced and applied to fit radio-occultation retrieved electron density profiles for altitudes above the F2-peak. The model considers two isothermal components representing the population of the O+ (ionosphere component) and the H+ (protonosphere component) ions. The purpose of the model is to achieve an accurate fit of the observed profiles in the topside ionosphere region while, at the same time, allowing a direct and simple derivation of two important ionospheric parameters, namely, the O+ vertical scale height and the upper transition height. Covering a time period of one year, the fits with the two-component model function are compared with those achieved with one-component functions commonly used in the literature and it is shown that the former provides significantly better fits than the later, with more than a factor of two improvement. The model predictions concerning: the correlation between the O+ vertical scale height and the upper transition height, the altitude dependence of the vertical scale height of the electron density, and the quantitative contribution of the protonosphere to the total electron content are examined and shown to be consistent with the observations and with previous studies. It is concluded that the model provides a realistic description of the vertical distribution of the two main ion constituents of the topside ionosphere.

Two research studies have been addressed in this thesis. Both of them are of actual scientific interest and are based on processing GNSS data. The first part of this thesis is devoted to GNSS detection and monitoring of solar flares. The second one is devoted to GNSS prediction of ionospheric Total Electron Content.
Regarding the first study, a new solar flare detector called SISTED has been designed and implemented. Its goal is to provide a simple and efficient way of detecting the most number of powerful X-class solar flares in real time operation. In addition, it can send early warning messages to prevent the harmful consequences of the increase of ejected particles from the Sun that may reach the Earth after a solar flare, especially in case of a Coronal Mass Ejection. The main benefit of SISTED regarding other detection techniques is that it does not require data from external providers out of the GNSS community. In addition, it can run in real-time operation and could provide value added data to GNSS users. The results show that SISTED was able to detect up to the 95% of the X-class flares reported by GOES for more than a half solar cycle.
Regarding the second study, a new approach to predict Global Ionospheric vertical TEC Maps has been designed and implemented in the context of the IGS Ionosphere Working Group. The motivation to develop a UPC Predicted product was the interest of ESA's SMOS mission. A recent application using UPC Predicted products is the generation of real-time global VTEC maps as background model. In addition, the predicted VTEC maps are used to generate the combined IGS Predicted products. The results obtained in this thesis show that the model performs well when the results are compared with those obtained by the other IGS analysis centers. In addition, applying the prediction model leads to better results than the use of time-invariant ionosphere for two days ahead.
In relation with this research, 4 publications in international journals indexed in JCR/ISI have been generated (and another one is under review process), and 7 presentations have been authored in international meetings, among the new UPC predicted product contributing to IGS, and the contribution to two competitive projects funded by the European Space Agency (AGIM and MONITOR).

This paper presents the features of the MONITOR project. This project initiated by ESA/ESTEC aims to increase the knowledge of the ionospheric effects and its impact on GNSS systems during active periods of solar activity. It includes the deployment of a set of GNSS-based ionospheric monitoring receivers worldwide distributed, the development of specific analysis software tools some of them integrated on a common platform, others distributed providing products routinely and a measurement campaign which will last beyond the peak of the current solar cycle

The paper reviews the current state of GNSS-based detection, monitoring and forecasting of ionospheric perturbations in Europe in
relation to the COST action ES0803 ‘‘Developing Space Weather Products and Services in Europe’’. Space weather research and
related ionospheric studies require broad international collaboration in sharing databases, developing analysis software and models
and providing services. Reviewed is the European GNSS data basis including ionospheric services providing derived data products
such as the Total Electron Content (TEC) and radio scintillation indices. Fundamental ionospheric perturbation phenomena covering
quite different scales in time and space are discussed in the light of recent achievements in GNSS-based ionospheric monitoring.
Thus, large-scale perturbation processes characterized by moving ionization fronts, wave-like travelling ionospheric
disturbances and finally small-scale irregularities causing radio scintillations are considered. Whereas ground and space-based
GNSS monitoring techniques are well developed, forecasting of ionospheric perturbations needs much more work to become
attractive for users who might be interested in condensed information on the perturbation degree of the ionosphere by robust indices.
Finally, we have briefly presented a few samples illustrating the space weather impact on GNSS applications thus encouraging
the scientific community to enhance space weather research in upcoming years.

This paper summarizes the main results obtained during the development of an Enhanced Precise Point Positioning (EPPP) Global Navigation Satellite Systems multifrequency user algorithm. The main innovations include the application of precise ionospheric corrections to facilitate the resolution of undifferenced carrier phase ambiguities, ambiguity validation, and integrity monitoring. The performance of the EPPP algorithm in terms of accuracy, convergence time, and integrity is demonstrated with actual GPS and simulated Galileo data. This can be achieved with very limited bandwidth requirements for EPPP users (less than 300 b/s for dual-frequency GPS data).